Various approaches have already been proposed to eliminate human operator reading errors and speed up the measurement process, and attempts have been made to develop machine-readable barcodes in different ways.
For instance, patent document U.S. Pat. No. 5,572,009 discloses a method of encoding a barcode in which a BP type of Code is formed from a P code with a given length, and whose half-bits and whole bits are alternately provided with light and dark symbols.
Patent document U.S. Pat. No. 5,742,378 discloses a barcode levelling rod having an alignment of the first and second (and third) scale dark symbols, the alignment being made in order cyclically at a constant pitch along the length of the staff. The first and second symbols have their widths made variable in different cyclic periods, and the third symbol has a constant width.
Patent documents U.S. Pat. No. 6,167,629 and U.S. Pat. No. 5,887,354 disclose levelling rods for electronic levelling, having a plurality of bar-shaped marks disposed adjacent to each other in a longitudinal direction along the levelling rod.
For all the above-mentioned levelling barcode rods, in near distance measurement, the position of the collimated barcode portion can be estimated by detecting the width dimension of the bar symbols. The width dimension of the bar symbols in the longitudinal direction of the rod is represented by a plurality of integers. If the distance between the levelling rod and the electronic level becomes large, the image of each bar symbol on the image receiver becomes small. As a result, it is difficult to discriminate accurate dimensions of the bar symbols. To this end, one plurality of barcodes disclosed in U.S. Pat. No. 5,572,009, U.S. Pat. No. 6,167,629 and U.S. Pat. No. 5,887,354 should be interpreted as two permutations of integers for near and remote distances. In these cases, the estimation of the position of the collimated portion is very coarse. It still requires a considerable amount of cross-correlation computation in a large size domain to determine the accurate position. For U.S. Pat. No. 5,742,378, FFT (Fast Fourier Transform) is used to calculate the distance between the rod and the electronic level in the remote distance measurement. Also, the height of the collimated portion is estimated by the detection of the phase angle of two periodic bar symbols. To obtain the accurate position, cross-correlation computation calculation is needed too.
Current barcodes for electronic levelling are monochrome. In the above-mentioned barcodes, the height readings are encoded by the dimension of the bars or spaces between the bars. Therefore, with current monochrome barcode scales, there are disadvantages in that it is very difficult to estimate the position (height and distance) of the collimated portion on the levelling rod with a simple decoding algorithm and a small number of calculation steps.